Why should we think that it will block the sun? Well, this joker's video, for one:
This based on NASA trajectories. But, does he have it right? This is what they say at NASA:
I've heard about three days of darkness because of Comet Elenin. Will Elenin block out the sun for three days?In other words, more hyperventilating about something which is simply wrong, by people who don't understand the science.
“As seen from the Earth, comet Elenin will not cross the sun’s face,” says Yeomans.
But even if it could cross the sun, which it can’t, astrobiologist David Morrison notes that comet Elenin is about 2-3 miles (3-5 km) wide, while the sun is roughly 865,000 miles (1,392,082 km) across.
How could such a small object block the sun which is such a large object?
Let's think about an eclipse of the sun, which happens when the moon appears between the Earth and the sun. The moon is about 2,500 miles (4,000 km) in diameter, and has the same apparent size as the sun when it is about 250,000 miles (400,000 km) away -- roughly 100 times its own diameter. For a comet with a diameter of about 2-3 miles (3-5 km) to cover the sun it would have to be within 250 miles (400 km), roughly the orbital altitude of the International Space Station. However, as stated above, this comet will come no closer to Earth than 22 million miles.